For microscopic investigations, the samples to be investigated are usually placed onto or under coverslips (also: “cover glass”), or into large-area sample chambers (well plates, microtiter plates, Labtech plates). A variety of coverslip thicknesses exist; in the case of the aforesaid sample chambers in particular, the coverslip thickness can fluctuate markedly. Different coverslip thicknesses negatively affect the optical performance of the system when the coverslip is arranged in the imaging beam path. In the context of a screening using a large-area sample chamber, a fluctuation in coverslip thickness results in diminished image quality (in particular, contrast as well as resolution are negatively influenced). This aberration, also referred to as “spherical error,” can be corrected by way of an additional adjustable correction lens element. This lens element is usually mounted inside the objective, the adjustment usually occurring manually via a so-called correction ring on the outer barrel of the objective. The focus that has been set is, ideally, left unchanged by the correction lens element.
DE 43 23 721 C2 deals with a microscope objective of this kind having a correction apparatus for adapting to different coverslip thicknesses; this document proposes that the correction mount having the correction lens element be both axially displaceable along the optical axis and radially rotatable around the optical axis. In particular, two such correction mounts are to be present. The proposed feature is said to make possible extremely uniform and jam-free displacement of the correction mount within the microscope objective. The objective proposed therein is said to enable correction for a coverslip thickness from 0 mm to 2 mm.
DE 10 2007 002 863 B3 likewise describes an adjustment apparatus, suitable for coverslip thickness correction, for microscope objectives, the adjustment apparatus being manually actuated. If coverslips of differing thickness are used in microscopic investigation, manual correction must be carried out again for each new coverslip thickness. This is time-consuming and makes the microscope more difficult to handle.
U.S. Pat. No. 7,593,173 B2 describes a motorized adjustment apparatus for coverslip thickness correction. This adjustment apparatus comprises a drive motor, installed on an objective turret, whose drive shaft is selectably couplable to one of several microscope objectives held on the objective turret. Each of the microscope objectives possesses a correction ring of the kind described earlier, which is to be brought into engagement with said drive shaft.
It has been found that in addition to the different coverslip thicknesses and/or coverslip thickness fluctuations addressed above, further parameters can contribute to spherical errors of the objective in the context of microscopic investigation. These parameters are, in particular, the nature and temperature of an immersion medium being used, as well as the material and structure of the coverslip. Coverslips are usually produced from float glass, which is further used in particular for LED covers. Such float glass exhibits differing flatness and homogeneity. Immersion media (immersion oil, water, or glycerol) are often introduced for various purposes between the microscope objective and coverslip, and the microscope objective is then used as an immersion objective whose front element is immersed into the immersion medium. Oil immersion objectives serve to increase the achievable resolution. Water immersion is often used for the observation of living cells or tissue, in order to prevent the prepared specimen from drying out. In general, immersion media can decrease contrast-reducing reflections due to a large change in refractive index at the interfaces. In the case of long-duration screenings of prepared specimens in particular, the temperature of the respective immersion medium changes during the investigation. This results in corresponding changes in image quality.
Aberrations resulting from the above-described effects will be defined hereinafter as “spherical errors.” The known methods for coverslip thickness compensation are not sufficient to eliminate all spherical errors.